Hearing of "fluorine" or "fluoride" most people nowadays think instantly
of dentistry, of dental caries or the different ways of fluoride application
to prevent that disorder. But these terms are not inventions of dentistry
and the use of fluorides originally was in no way related to that profession.

Fluores - lapides igni
liquescentes

In the 16th century, when Nostradamus demonstrated his prophetic capabilities
and Paracelsus expressed his view that the dose alone makes a thing a poison,
another contemporary and professional colleague was interested in metallurgical
affairs: the German physician Georg Bauer, of Chemnitz. Under his latinized
name "Georgius Agricola" he is known in the history of medicine as the man
who very aptly described the diseases of the miners of his time (1), diseases
which he still ascribed to evil ghosts doing mischief in the mines. This
was but a very small aspect in his "De re metallica", the first written detailed
description -fully in Latin- of how to prepare metals from ores and which
soon became translated into many languages (2). It even was published in
Chinese (3). In his work Bauer explains how admixture of fluxes ("lapides
igni liquescentes (fluores)" = lat.: "stones which become liquid in fire
(flows)") facilitates the smelting of ores. Fluxes work as a solvent for
ores that would otherwise need much more heat (and thus energy) to become
liquid for further processing. The aid, called "Flußspat" (fluorspar)
by the German miners, had for the first time been mentioned by Basilius
Valentinus towards the end of the 15th century (4), and was more extensively
described in one of Bauer´s early works in 1530 (5). Fluorspar occurs
in nature in several beautifully colored varieties. The violet variety
("fluores colore violaceo") looks like amethyst, the green one
("fluores viridi") resembles emerald and so these varieties
were occasionally sold instead of the gems (hence the name "false amethyst"
or "false emerald" for fluorspar).

Attack on glass

If to a sample of fluorspar some acid, e. g. sulfuric acid, is added and
the mixture is gently heated, toxic fumes develop which attack glass. There
are rumors that already in 1670 a Nuremberg glassworker, Heinrich Schwanhard,
made use of this reaction for artistic etchings on glass (6-8), but this
could not be verified; Schwanhard rather used nitric acid on a softer variety
of glass, as a review by Partington (9) revealed. The first account
of the fluorspar process was published by John George Weygand in 1725, the
recipe being given him by Matthäus Pauli, of Dresden, who in turn had
become aware of this secret knowledge of an unknown English glassworker around
1720 (9). The first, though superficial, examination of the chemical reaction
was published by Andreas Marggraf in 1768 (4).

Identification of
fluorspar

Inspired by Marggraf´s work, the Swedish apothecary
Carl Wilhelm
Scheele started, in 1771, a systematic investigation to find
out the chemical nature of fluorspar and the details of its reaction with
acids. He too observed the attack of glass by fumes which he obtained by
slowly heating, in a glass retort, a mixture of fluorspar and sulfuric acid.
The solid residue in the retort, extracted with water, revealed the presence
of lime upon addition of ammonia. The fumes, if led into water, released
a white mass identified as silica. The resulting solution showed an acid
reaction. Scheele called it "Flußspatsäure" (acid of fluorspar,
fluoric acid). On adding it to lime water a white precipitate was formed
which showed the same behavior as fluorspar, i.e. fluoric acid could again
be released upon treatment of the filtered and
dried precipitate with sulfuric acid (10).

It is thus Scheele´s work which showed means to identify fluorspar (then
also called "fluate" of lime):

· the etching of glass
by fumes developed after addition of some acid to the sample,

· release of silica if the fumes, after
contact with glass, are led into water,

· precipitation of fluorspar if the fumes
-or solutiions of the fumes- are led into lime water.

The unique ability to etch glass, which proves the presence of a fluoride
in a sample, led to the incidental detection of fluorides in samples wherein
it possibly would have never been sought. Thus, the physician and chemist
Jacob Berzelius, in 1822, found fluoride in a water sample (Carlsbad water)
the residue of which he treated with nitric acid in a platinum baker which
he covered with a glass that finally became etched.

The chemical
nature of the acid of fluorspar

Even though the name "fluoric acid" (fr.: "acide fluorique", ital:
"acido fluorico") applied to the new acid discovered by Scheele clearly reminds
us today of the element "fluorine" bound in it, nothing was known by the
time about the chemical nature of this acid, the name being merely derived
from the latin term "fluores" for fluorspar. After Scheele´s first
experiments, some researchers raised doubts whether it really is a new acid,
but claimed it might be simple muriatic acid or an acid derived from the
sulfuric acid used in the experiments. Years after Scheele had shown that
the acid can also be made by treatment of fluorspar with phosphoric
acid or nitric acid (10-12) it was still supposed by some researchers
that it might be formed by a modification of phosphoric acid which often
accompanies it in natural products (rock phosphate, fossil bone samples)
(13, 14).

A
new element - fluorine

When muriatic acid was identified as a compound of the newly discovered element
chlorine with hydrogen (which fact also showed that oxygen [greek: "acid
forming"] is not an essential component of acids, as was believed until then)
fluoric acid was suddenly considered to be a hydrogen compound of another
new element with properties similar to chlorine. The name "fluorine"
at first was proposed for the new prospective element, later the term "Phtor"
(greek for "destructive") seemed to be more appropriate because of the
destructive properties of its compounds. But "Phtor" was accepted only in
the eastern hemisphere (15).

The isolation of the new element, to demonstrate its existence, occupied
many researchers for a long time. A first step was the preparation of a
concentrated hydrofluoric acid by Thénard and Gay-Lussac (15,16).
Their product fumed strongly in air, rapidly dissolved glass, and caused
extraordinary burns on contact with the skin - a phenomenon the
authors described in some detail. Other researchers experienced even more
toxic effects, some had to pay a high price (severe disease or even
loss of life) for their -finally unsuccessful- attempts
to isolate the element, as they were not careful enough
in handling the hydrogen fluoride (HF). Thénard and Gay-Lussac
also described fluoboric acid, a new complex acid similar
to fluosilicic acid. When the famous chemist Berzelius summarized, in
1824, his experiments with a series of new fluorine compounds, many
more fluoride-bearing natural minerals had alreday been discovered
(e. g. apatites, cryolite, hornblendes) by chemists of the time, and
sodium and potassium fluorides had already been prepared. Berzelius
then showed that the fluorides
of ammonium, magnesium, beryllium, aluminum, cadmium,
copper, lead, tin, antimony, uranium, and many other metals
differ from their salts of any other acids known at the
time, and thus confirmed that fluoric acid bears in fact
a new radical (16a). But its isolation still posed a challenge.

Until the French chemist
Henri
Moissan prepared elemental fluorine in 1886 (16), quite a number
of researchers got an example of resistance by the way how vigorously it
resisted any attempts to get it in the free state. Moissan
prepared elemental
fluorine by electrolysis (in a specially
tightened apparatus) of liquefied, water-free hydrogen fluoride to which
some potassium bifluoride had been added to increase electric conductance.
Beginning shortly afterwards (17), a number of
patents were filed on methods for
the electrolytic production of elemental fluorine. Frank C.
Mathers and C. O. Anderson (who later became president of the Ozark-Mahoning
Company) conducted their electrolysis experiments in cooperation with the
Chemical Warfare Service (17a) and published their results after World War
I. Furthermore, it was Mathers´ interest in fluorine chemistry which
led -somewhat later- to another "great discovery". In Mathers´ set of
chemicals, Joseph Charles Muhler found a bottle of stannous fluoride which
became his favored "caries-preventive" additive to toothpaste. This discovery
enabled him to continue his fluoride research work in a newly erected dental
institute at the University of Indiana, i. e. in "the house that CREST built".

However, fluorine´s high reactivity and rather uncontrollable
behaviour in certain reactions prevented any widespread uses of the
element in the chemical industry for a long time.
Most of its compounds were prepared, therefore, by tricky
indirect reactions involving fluorides.

Properties of
fluorine

Fluorine, a yellow gas (18), is the most electronegative and the
most reactive chemical element. It reacts with nearly all organic and
inorganic materials, even with gold and platinum. Hydrogen and fluorine
react with explosive violence. With water, fluorine forms hydrofluoric acid
and ozone. A jet of fluorine from a pressure container reacts with human
flesh and can cause extremely severe burns that are very difficult to
heal. Mechanisms of tissue destruction include destructive oxidation
by fluorine, thermal damage from the heat of reaction, and tissue poisoning
by HF formed. "HF is formed by the reaction of fluorine with moisture on
the skin. HF is a protoplasmic poison with great penetrating power and causes
deep-seated burns that heal very slowly." Even the salts of fluorine
("fluorides") which collect on the inside of pipes, valves and other equipment
are extremely dangerous when inhaled or ingested, therefore "every precaution
should be taken to avoid breathing or swallowing them" (19).

Military uses
of elemental fluorine

During World War II fluorine was used for very special purposes:
the Fiat Final Report No. 838 on "Elemental Fluorine" states that since 1940
Germany´s IG Farbenindustrie operated several fluorine electrolysis
cells. One of them was built at Gottow during the war for a secret project
of the Oberkommando des Heeres (German Army High Command). It "represented
the largest production of elemental fluorine in Germany. The fluorine
was reported to have been produced solely for the manufacture of a new incendiary
agent, chlorine tri-fluoride" (20). An important finding in the research
at Leverkusen "was the explosive property possessed by liquefied or compressed
fluorine which had not been properly purified. In their initial work on storing
elemental fluorine under high pressures a few explosions of the steel cylinders
occurred. The explosive property of the fluorine was attributed to traces
of such impurities as O3, F2O2,
OF2 and ClOF, which were present in amounts too small to be
determined."

In the United States, fluorine was used during World War II for the
production of uranium hexafluoride, which, at elevated temperature,
is a gas that may be used for the separation of uranium isotopes.
The enriched radioactive uranium was used for the construction of the
first atomic bombs which went down on Hiroshima and Nagasaki in 1945
(21,22). Uranium refining for nuclear energy is still one
of the major uses for elemental fluorine.

Too reactive
to stay free

Given the high reactivity even in the presence of traces of impurities,
it is obviously impossible to keep elemental fluorine in a free elemental
state -except for a fraction of a second- under natural conditions.